The present disclosure relates to optical waveguide devices made from deuterated amorphous carbon, to a process for the preparation of the optical waveguide device, and to its use.
High-bandwidth communications generally rely on optical fibers and optical components for signal enhancement, and signal-routing as well as for adding and dropping information. The requirement for high capacity communication systems has led to the use of optical waveguides. The operation of optical waveguides is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
An optical waveguide generally consists of a core made of a high-refractive index material and a cladding of a low-refractive-index material surrounding the core. In traditional optical waveguide technology, the core and cladding are made of doped silica glass, where the refractive index contrast, i.e., the difference of the refractive index of the core layer relative to the cladding layers, is achieved by a doping profile, e.g., using P-doping or Ge-doping.
Planar waveguides are typically comprised of layers of low loss optical materials of precise indices of refraction. Both step index and gradient index waveguide structures are known in the art. For planar polymer and glass waveguides, in particular, step index structures are most easily achieved through successive coating of materials with differing indices of refraction. Typically, a waveguide core has a refractive index that is 0.3% to 2% higher than a cladding material. The magnitude of this refractive index difference (Δn) is set to optimize the performance of the planar waveguide or to match light modes when the transition is made from the planar device to an optical fiber. A higher refractive index contrast generally provides lower absorption losses.
A typical technique for fabricating the core structure of an optical waveguide device is by means of a plasma enhanced chemical vapor deposition (PECVD) process. The PECVD process utilizes gaseous precursors for depositing the desired materials. For deposition of silicon dioxide, the precursor gases typically include silane and nitrous oxide. For deposition of silicon oxynitride, the precursor gases typically include silane, nitrous oxide, and ammonia. The resulting core structures from these materials, however, have a large hydrogen concentration incorporated in the form of Si—H groups and/or N—H groups (in the case of silicon oxynitride). These groups and fragments introduce additional absorption into the optical transmission. For example, the first overtone of the Si—H induced absorption lies at about 1510 nanometers (nm), which overlaps with the spectral window that is commonly used for optical signal transmission. The first overtone of the N—H and the second overtone of the Si—H bond cause losses around 1,500 nm wavelength. The spectral window that is commonly used for optical signal transmission extends from about 1,540 nm to about 1,570 nm, hereinafter simply referred to as the desired optical transmission window. This window has been chosen for optical transmission due to the low costs associated with fabrication of lasers at these wavelengths, that the optical transmission losses for silicon dioxide based waveguides are minimal around 1,550 nm (due to purity of fabrication), and that for all-optical amplifiers in fiber networks (avoiding electrical-optical conversions) the only currently available amplifiers are based on erbium-doping.
Plasma-enhanced chemical vapor deposition (PECVD) techniques allow for optical processing elements to be directly incorporated on a substrate. During PECVD, gaseous precursors are exposed to plasma to form fragments and elements of the gaseous precursors, which are then deposited onto the substrate. A significant advantage of PECVD processes is that lower temperatures can be employed relative to conventional CVD processes. However, there can be significant tradeoffs to using PECVD techniques, including those adapted for low temperature operation. In conventional PECVD techniques, nitrous oxides and silane are typically utilized for deposition of silicon oxide. Unfortunately, as previously noted, such elements often result in a material having a high level of hydrogen absorption at wavelengths around 1,500 to 1,600 nanometers. In order to remove the chemical constituents that give rise to this loss, it may be necessary to heat (i.e. anneal) the deposited material to higher temperatures, thereby eliminating the advantages provided by the low temperature deposition process.
The losses are reduced to lower values by higher anneal temperatures or longer anneal durations, but both routes bear the risk of introducing unwanted defect-scattering or crystallization of the amorphous materials. A further negative side effect of the high-temperature annealing is the introduction of anisotropic stress, caused by the difference in thermal expansion coefficients between the silicon substrate and the material of the waveguide. The stress correlates with the temperature difference between room temperature and the highest annealing temperature or a material-specific temperature close to its annealing point, whichever is lower. The stress gives rise to birefringence and polarization-dependent transmission characteristics and is not desired for most applications.